Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer (EML) emits light may generally be readily tuned with appropriate dopants.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules. In general, a small molecule has a well-defined chemical formula with a single molecular weight, whereas a polymer has a chemical formula and a molecular weight that may vary from molecule to molecule. As used herein, “organic” includes metal complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic opto-electronic devices. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may comprise a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent. Side emitting OLEDs may also be fabricated, and one or both electrodes may be opaque or reflective in such devices.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processable” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level. U.S. Pat. No. 6,893,743 discloses an OLED, comprising a substrate and a light-emitting layer sandwiched between an anode and a cathode on the substrate. The light-emitting layer comprises at least a host material, having electron-transporting or hole-transporting properties, a compound A, capable of phosphorescent emission at room temperature, and a compound B, capable of phosphorescent emission of fluorescence emission at room temperature, and having a maximum light emission wavelength longer than the maximum light emission of compound A. The maximum light emission of the OLED is attributable to compound B, and the light emission attributable to compound B is intensified by compound A to elevate the light emission efficiency. Where compound B is a fluorescent compound, aging deterioration of the OLED can reportedly be prevented. However, the patent does not disclose an OLED in which all or substantially all of the emission of the OLED is produced by compound B.
U.S. Pat. No. 6,515,298 discloses OLEDs, comprising intersystem crossing agents. In one disclosed embodiment, the fluorescent efficiency of a fluorescent emitter is enhanced by combining the fluorescent emitter with a phosphorescent sensitizer, where the phosphorescent sensitizer operates as an intersystem crossing agent. In a second disclosed embodiment, the phosphorescent efficiency of a phosphorescent emitter is enhanced by combining the phosphorescent emitter with a host and intersystem crossing agent, where both the host and agent both have singlet spin multiplicity. In a third disclosed embodiment, a thin layer of ISC agent is placed between the HTL and ETL of the OLED, where the ISC agent has an optical absorption spectrum that strongly overlaps the emission line of the material found at the site of recombination. Enhancement of the efficiency of a phosphorescent material is not disclosed.
D'Andrade et al., High Efficiency Yellow Double-Doped Organic Light-Emitting Devices Based on Phosphor-Sensitized Fluorescence, Applied Physics Letters 79 (2001) pp 1045-1047 discloses high-efficiency fluorescent OLEDs utilizing a phosphorescent sensitizer. Double doping of fluorescent red (DCM) and phosphorescent green (Irppy) sensitized the fluorescent red emission by the phosphorescent green emitter, enhancing the fluorescent red OLED efficiency.
Feng, et al., Proceedings of SPIE—The International Society of Optical Engineering, 4105 (2001) pp 30-36 disclose double-doped fluorescent red and blue emitters for the generation of white (a mixture of colors) emission.
Kawamura et al., Journal of Applied Physics, 92, 1, (2002) pp 87-93, disclose white devices utilizing three blue, yellow, and red phosphorescent emitters in a polymer PKK host.
D'Andrade et al., Electrophosphorescent White-Light Emitting Device with a Triple Doped Emissive Layer, Advanced Materials, 16, 7, (2004) pp 624-628, and Controlling Exciton Diffusion in Multilayer White Phosphorescent Organic Light Emitting Devices, Advanced Materials, 14, 2, (2002) pp 147-151, disclose mixtures of two or three phosphorescent emitters in a vacuum deposited OLED in which all of the deposited emitters contribute to the device white emission.